TEOS (tetra-ethyl-ortho-silicate) is a silicon-containing compound that is a liquid at room temperature. TEOS is used in many applications to deposit dielectric film on a substrate. TEOS is used in applications where conformality is required as silicon dioxide (or “TEOS oxide”) films deposited by TEOS chemical vapor deposition processes have good conformality. TEOS oxide is often deposited by a plasma enhanced deposition chemical vapor deposition (PECVD) process. TEOS-based PECVD processes typically involve exposing a substrate to a process gas including TEOS and an oxidant such as oxygen or ozone. Liquid TEOS is first vaporized in a preheater to about 150 degrees Celsius. TEOS vapor flows first to a “mixing bowl” in the deposition chamber, then to the individual showerheads in each station—if a multi-station chamber is used.
In a Multi-station Sequential Deposition (MSSD) system, the wafer spends equal amount of time at each of multiple stations. As an example, at each station, the wafer experiences more or less the same process as follows: the wafer is heated on the pedestal with helium purge; some oxygen flows into the station out of the showerhead; the precursor gas flows out of the showerhead with some oxidant gas; plasma is struck; a dielectric film is deposited onto the wafer; the precursor gas is turned off; then a few second later, the oxidant gas and the plasma are turned off; and lastly, the wafer advances to the next station. Gases flow to the showerheads from a mixing bowl. Every station receives the same gases.
To improve the throughput, the first station may be dedicated to heating the wafer, commonly known as the temperature soak step. This reduces the time a wafer spends at each station by eliminating the initial temperature soak step from each station. The wafer is thereby heated at the first station for about the length of time the other stations run their processes. Because the wafer does not lose much heat between stations, the temperature soak only needs to be performed once. Therefore, more wafers per hour can run through the system because each wafer spends less time in the system. Of course, the oxide that would have been deposited at the first station must now be deposited at other stations.
As throughput increases and film thickness decreases, the amount of time at each station also decreases. Not only is deposition time decreased, but also the temperature soak time. At very short temperature soak times, the wafer is not heated long enough to reach a high temperature, resulting in small bin defects. FIG. 1 shows the relationship between defect level on the y-axis, shown as particle adders with SiN Cap, and soak time/wafer temperature. In the particular experiment used to generate the data in FIG. 1, SiN is deposited on wafers to magnify small bin defects for optical detection. Because SiN has poor conformal properties, when deposited on a wafer it highlights defects that ordinarily may not have been visible by some optical detection methods.
As soak time increases, defects of all sizes decrease, but the smallest defects are most sensitive to soak time. Small bin defects are those smaller than 0.16 microns. As device size shrinks to 65 nanometers and below, small bin size defects can greatly reduce yield. Even more yield problems can arise as the number of integration layers increase; the upper layers magnify small bin defects from lower layers to become large bin defects.
Because small bin defects are strongly correlated to wafer temperature, one solution is to lengthen the temperature soak time. The wafer would reach a high temperature, and the small bin defects would be eliminated. However, the advantage of high throughput would be lost. What are needed therefore are improved methods for depositing thin TEOS films at high throughput without small bin defects and without sacrificing the film quality.